Telescope optical systems are designed to focus light from a distant object occupying a finite field of view onto an image focal surface. High definition, high resolution, sharp images require a high spatial frequency content in the image. However, the frequency content can be degraded by optical aberrations which blur the image, increasing the image spot size thereby decreasing image resolution. Telescope optical systems comprise optical elements which focus light while simultaneously attempting to eliminate optical aberrations of the image, including spherical aberration, coma, astigmatism, field curvature and distortion. Two broad categories of optical systems have been used to collect and focus light from a distant object. An all-refracting system may comprise a negative diverging lens disposed between two positive converging lenses, whereas an all-reflective system may comprise a negative convex surface disposed between two positive concave surfaces. An all-reflective system uses mirrors to reflect light in desired directions while an all-refractive system uses lenses to accomplish the same imaging function by refracting the incoming light, as is well known.
All-reflective mirror systems have two principal advantages over all-refractive lens systems. All-reflective systems have geometric light deviating characteristics which are the same for light of any wavelength, so that imaging performance is not affected by chromatic aberrations as with refractive systems. All-reflective systems are therefore preferred for imaging applications which span a broad range of operating wavelengths because they are not affected by chromatic aberrations.
All-reflective systems also have mirrors that can be made in any desired size while the sizes of refractive systems are limited by the availability of high-quality transmissive materials. Large refractive optical systems may in principle control optical aberrations but may still be impractical to build due to the difficulty and expense of producing the required lenses. Large reflective optical systems tend to minimize the expense of manufactured optical elements while also controlling optical aberrations. Large reflective mirrors are easier to construct and are lighter in weight as compared to large refractive lenses. As the size of the telescope apertures increase, for example, above 30 cm, reflective systems are increasingly favored because of limits on the size and quality of refractive materials.
Diffraction limited image quality is a well known goal of optical systems. For reflective mirrors, the angle of incidence equals the angle of reflection except near the edges of a finite mirror aperture. In any finite aperture system, wave phenomena will limit the resolution due to the size of aperture, which truncates the wavefront causing diffraction. Each image point has a finite spatial extent which affects the sharpness of an image, that is, the contrast at the boundaries between the dark and light elements of the image. Diffraction reduces the abruptness of the transitions from dark to light, which is perceived as a degradation of image contrast. The finite aperture size dictates a finite frequency response causing a progressive reduction in contrast as spatial frequency increases. If all telescope anomalies were eliminated, there would still be diffraction because the telescope has a finite size which causes truncation of the wavefront. Hence, the finite size of the telescope aperture causes the image point to grow in size. The image point will not be a perfect image point but a blurred and enlarged image point. Hence, the edge diffraction of the aperture limits the resolution of any telescope. It is generally desirable that any telescope be diffraction limited to achieve the highest resolution possible for a given finite aperture size.
One difficulty with all-reflective systems is achieving good image quality over a wide field of view while minimizing the sizes of individual mirrors and the volume of the telescope package containing the mirror elements. A single mirror with an appropriately shaped surface is capable of forming a perfect geometric image of a single object point. The mathematical description of the surface of the mirror is dictated by the fundamental requirement that the length of any path from the object to the image be equal to the length of any other similar path. For an infinitely distant object point, for example, the theoretical figure which achieves perfect geometric imagery is the paraboloid of revolution. The ideal mirror shape produces the best possible geometric image quality for a given pair of object and image conjugate points. Errors in fabrication distort the actual mirror surface from the ideal shape and cause the size of the geometric image to grow, degrading the frequency content of the image. Even in the absence of errors, diffraction resulting from the finite size of the collecting mirror aperture places an ultimate lower diffraction limit on the size of the image spot for a given system. When the geometric image spot size from the ideal mirror shape with fabrication errors is smaller than the spot size caused by diffraction, the system is said to be diffraction limited. Optimum telescope systems operate near the diffraction limit to provide the highest resolution possible with the least degradation of the frequency content.
An extended object can be considered as a continuum of object points each of which is subject to distortion from perfect imaging by diffraction and optical aberrations. A single mirror surface is generally not capable of perfect imaging for more than one object point and image point. Except in special, impractical cases, a single mirror can not form a perfect image of extended objects. Hence, optical systems must add additional reflective surfaces to provide near perfect imaging of extended objects. Additional surfaces provide additional degrees of freedom which define the shapes and locations of mirror surfaces. Thus, a multiple mirror system has a set of surfaces and spacings defining path lengths traversed by rays propagating from the object to the image. The path lengths for each point are equal for enough object points to span the required angular field of view. A compromise is reached between less that perfect imagery and manufacturability of the reflective surfaces. There is design latitude that does not significantly affect image quality because geometric spots need only be smaller that the diffraction limit. Design difficulty increases with an increase in the field of view of the object to be imaged because of the difficulty in maintaining equal path lengths for an increasing number of conjugate points. Additional mirror surfaces and more complex mirror surface shapes have been used to meet the demands of high quality imaging of large extended objects. The art of optical design consists of finding the best overall compromise between the performance and complexity of an imaging system. Obtaining good image quality over a wide field of view, for example, greater than 2.5 degrees, is difficult in a multiple mirror, all-reflective system. The task is simplified if the field of view is restricted to a one-dimensional line in object space, rather than a two-dimensional circular or square field. This linear field can be scanned to build up the image of a two-dimensional extended object over time.
Aspheric surfaces, which cannot be represented as part of a large sphere, include conic sections such as hyperboloids, paraboloids, ellipsoids and oblate spheroids. A conic section is one of several possible shapes derived from an intersection of a plane with a cone. There are also general aspherics, for which the shape of the surface is represented by a general polynomial equation in one of several established forms. The use of aspheric surfaces is well known. For example, ellipsoidal and hyperboloidal mirrors are described in U.S. Pat. Nos. 4,101,195 and 4,226,501. Both mild and strong aspherics have been used and are characterized by the extent of departure from spherical. Aspheric surfaces are more difficult to manufacture than spherical surfaces and are more complicated to design as part of an optical system. Spherical mirrors are always symmetric because any section of a spherical mirror is the same as any other section. This property makes spherical mirrors easy to manufacture as compared to aspheric mirrors. Aspheric surfaces can, however, replace several spherical surfaces for the purpose of reducing aberrations. Between conic sections and general aspheres, the former are preferred because they are usually easier to fabricate and test.
One prior art telescope with a wide linear field of view is a three mirror anastigmat reflective triplet shown in FIG. 1 herein and described in U.S. Pat. No. 4,240,707. FIG. 1 is a cross-sectional view of image rays, focal surface and mirrors for one object point in a linear field of view. The linear field of view extends orthogonally to the cross section of the focal surface, mirrors and image rays of FIG. 1. In this prior art form, high resolution imaging is achieved with an aperture stop centered on a convex secondary mirror and with concave primary and tertiary mirrors eccentric sections of larger symmetric parent reflective surfaces. The surfaces are typically aspheric, including for example, conic sections, depending on the field of view and image quality desired. With conic aspherics, linear fields of view of up to five degrees on a flat focal surface have been demonstrated. With general aspherics, linear fields up to 15 degrees have been demonstrated. This reflective triplet operates most effectively at focal lengths from three to six times the aperture size; that is, with focal ratios of f/3 to f/6. The focal ratio, often called the f-number, is the ratio of the focal length divided by the aperture diameter. In FIG. 1, the distance from the tertiary mirror to the focal surface is approximately equal to the effective focal length. For this type of telescope system, the overall system length, as characterized by the distance from the tertiary mirror to the focal surface, is typically comparable to the system effective focal length. Long focal lengths are desired for high resolution imaging but are disadvantageously limited by practical sizes of the optical system.
Producing a high resolution image is only one of several desired design goals of a complete optical system. The image must also be detected and recorded to be useful. Optical imaging systems produce high resolution output in part by sampling their images with a large number of detector elements, or pixels. The detector pixel must be small relative to the image size. A large number of samples may be achieved by reducing the pixel size or by increasing image size relative to the pixels by increasing the focal length of the telescope. There is a lower technology limit to detector pixel sizes. For charge coupled device (CCD) type detectors, this limit is presently at about 7 micron pixel size. Once this limit is reached, increased resolution can only be achieved by increasing image size, which requires an increase in focal length. In an all-reflective triplet of the type shown in FIG. 1, one way to increase the focal length of the telescope is by scaling up the whole optical system while maintaining the relative proportions between the mirror shapes, sizes and spacings. This increases the size of the image relative to the detector pixels, producing more samples across the image for increased resolution. However, this approach results in a large package length, weight and volume when the end requirement is a system with a very long effective focal length. An all-reflective triplet with a large effective focal length may be particularly undesirable in applications requiring launch into space, where large physical sizes are prohibited by considerations of weight and cost.
Starting from the prior art reflective triplet of FIG. 1, another way to increase the effective focal length is to redistribute the converging and diverging powers of the three mirrors while maintaining the original spacing between the secondary mirror and the primary and tertiary mirrors. An example of such a resulting system is shown in FIG. 2. The focal length of this optical system may be, for example, 12 times the aperture, giving a focal ratio of f/12. The package size containing the three curved mirrors remains the same. However, the long effective focal length still requires that there be a large spacing between the tertiary mirror and the focal surface, as shown. This long optical path could be folded with a series of flat mirrors in order to package the system within a smaller volume. Such mirrors would, however, complicate the system and add significantly to the weight of the system and so would be disadvantageous for space applications. A further disadvantage to this approach is that the surface of best image quality is typically no longer flat, but follows a curved profile. This makes fabrication of the detector array more difficult, although a mild curvature can be approximated by a series of short, flat array segments distributed along an ideal profile.
FIGS. 1 and 2 demonstrate the difficulties encountered in trying to adapt the prior art three mirror anastigmat design form to applications requiring a long effective focal length. Either the packaged volume must become disadvantageously large, or a number of otherwise unnecessary mirrors must be added to fold the optical path. Either approach is undesirable fox applications which place a premium on system size and weight, for instance, those systems launched into space.
Four mirror reflective systems are also known, for example, the one disclosed in U.S. Pat. No. 5,142,417 at FIG. 2. This optical system advantageously describes an f/12 to f/20 optical system with a large effective focal length. This system also advantageously teaches spherical mirrors for ease of manufacture, but disadvantageously teaches disjointed mirror axes between the mirrors and a disadvantageously resulting relatively small angular field of view. This mirror system adds a fourth mirror to increase the focal length but adds additional telescope design costs and manufacturing complexity. The disjointed mirror axes particularly increase system complexity and alignment requirements, presenting difficult manufacturing challenges. These and other disadvantages are solved or reduced using the present invention.